metalness map in cycles?
You can assign several materials to a mesh and a material can be used by several meshes.

In this first image, 2 materials (1) are assigned to the left object (2), and the wood part has its UVs located in (3) in the texture.



In this second image, again 2 materials are assigned (1) but to the right object (2), and its UVs are located in (3).



To assign several materials, use the '+' button on the right, so that a new material slot appears.
Then assign (or create) a material for this slot.
Last step, go to edit mode, select the wanted faces and click 'assign' in the material part :